Electrocatalytic tube of electrochemical-catalytic converter for exhaust emissions control

ABSTRACT

An electrocatalytic tube for controlling exhaust emissions, which adopts to purify the exhaust, comprises a tube, an anode layer and a cathode layer. The tube is composed of a solid-state electrolyte layer. The solid-state electrolyte layer includes an enclosed chamber, an inner wall formed inside the enclosed chamber, and an outer wall formed outside the enclosed chamber. The enclosed chamber has a sub-atmospheric reducing environment. The anode layer and cathode layer are respectively coated on the inner wall and outer wall of the solid-state electrolyte layer. The reducing environment facilitates an electromotive force to occur between the anode layer and the cathode layer. The electromotive force promotes catalytic decomposition of nitrogen oxides of the exhaust on the cathode layer.

FIELD OF THE INVENTION

The present invention relates to an electrochemical-catalytic converter,particularly to an electrocatalytic tube of theelectrochemical-catalytic converter for controlling exhaust emissions.

BACKGROUND OF THE INVENTION

Fresh and clean air is essential for human health. Science andtechnology has promoted economical development. However, the exhausts ofvehicles and factories, especially motor vehicles and heavy industryfactories, seriously pollutes the air.

The emission standard of motor vehicles has been increased persistently.However, the continuously increasing motor vehicles still bring aboutmore and more serious air pollution. In a motor vehicle, the enginethereof burns fuel and converts chemical energy into mechanical energy.The burning process of fuel generates the polluting constituents,including nitrogen oxides (NO_(x)), carbon monoxide (CO), hydrocarbons(HCs), and particulate matter (PM), which would form photochemical smog,deplete ozone, enhance the greenhouse effect, cause acid rain, damagethe ecological environment and finally danger human health.

Carbon monoxide comes from incomplete combustion. The capability ofcarbon monoxide to combine with hemoglobin to form carboxyhemoglobin(COHb) is 300 times higher than the capability of oxygen to combine withhemoglobin to form oxyhemoglobin (HbO₂). Therefore, too high aconcentration of carbon monoxide would degrade the capability ofhemoglobin to transport oxygen. Nitrogen oxides are generated by thecombination of nitrogen and oxygen and mainly in form of nitrogenmonoxide (NO) and nitrogen dioxide (NO₂). Reaction of nitrogen oxidesand hydrocarbons is induced by ultraviolet ray, generating poisonousphotochemical smog, which has a special odor, irritates eyes, harmplants, and reduces the visibility of the ambient air. Nitrogen oxidescan react with water in the air to form nitric acid and nitrous acid,which are the constituents of acid rain. Hydrocarbons can irritate therespiratory system even at lower concentration and will affect thecentral nervous system at higher concentration. Particulate matter candanger human health and can even cause cancer.

Therefore, many nations, including EU, USA, Japan and Taiwan, haveregulated stricter emission standards for nitrogen oxides, carbonmonoxide, hydrocarbons and particulate matter, such as BIN5 of USA andEURO 6 of EU, which not only regulate the emissions of the pollutingconstituents but also encourage the manufacturers to develop, fabricateor adopt the newest pollution control technologies and apparatuses.

A U.S. Pat. No. 5,401,372 disclosed an “Electrochemical CatalyticReduction Cell for the Reduction of NO_(x) in an O₂-Containing ExhaustEmission”, which is dedicated to removing nitrogen oxides, wherein anelectrochemical-catalytic reducing reaction and a vanadium pentaoxide(V₂O₅) catalyst convert nitrogen oxides into nitrogen. However, theprior art needs an electric source to power an electrochemical cell.Therefore, the prior art consumes power and cannot eliminate otherpolluting constituents simultaneously.

A U.S. patent application Ser. No. 13/037,693 disclosed an“Electrochemical-Catalytic Converter for Exhaust Emission Control”,which can eliminate nitrogen oxides (NO_(x)), carbon monoxide (CO),hydrocarbons (HCs), and particulate matter (PM) in the exhaust, andwhich comprises an electrochemical module, wherein the nitrogen oxidesare decomposed into nitrogen and oxygen, and wherein carbon monoxide,hydrocarbons, and particulate matter are converted into water and carbondioxide by an oxidation catalyst. Therefore, the prior art can eliminatemultiple polluting constituents simultaneously.

However, the abovementioned “Electrochemical-Catalytic Converter” needsa reducing gas system to generate an electromotive force, whichincreases the fabrication cost. Further, the circulating reducing gasheated by a heating unit will expand and contract, which is likely todamage the structure of the anode. Besides, the“Electrochemical-Catalytic Converter” is hard to fabricate into acompact structure for vehicle application. Therefore, the prior artstill has room to improve.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to overcome theproblems of the conventional electrochemical-catalytic converter,including high fabrication cost, expansion and contraction-inducedstructural damage, and bulky volume.

To achieve the abovementioned objective, the present invention proposesan electrocatalytic tube, which comprises a tube, an anode layer and acathode layer. The tube is made of a solid-state electrolyte layer,which includes an enclosed chamber, an inner wall inside the enclosedchamber, and an outer wall outside the enclosed chamber. The enclosedchamber can have a sub-atmospheric reducing environment. The anode layerand the cathode layer are respectively coated on the inner wall and theouter wall of the solid-state electrolyte layer.

The exhaust passes over the cathode layer for treatment. For the presentinvention, the exhaust is from lean-burn engines and thus is at anoxidizing environment. The reducing environment of the enclosed chamberand the oxidizing environment of the cathode layer induce anelectromotive force to form between the anode layer and the cathodelayer to promote the decomposition of nitrogen oxides. The oxidizingenvironment over the cathode layer enables the oxidation of carbonmonoxide, hydrocarbons, and particulate matter of the exhaust.

In the present invention, the electrocatalytic tubes can be assembled toform a ceramic monolith to be an advanced electrochemical-catalyticconverter. Thus, the present invention has simple structure and lowerfabrication cost than the prior art of the conventionalelectrochemical-catalytic converter. Besides, the sub-atmosphericreducing environment exempts the present invention from structuraldamage caused by thermally-induced expansion and contraction. Therefore,the electrocatalytic tube of the present invention can have a longerservice life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing an electrocatalytictube according to a first embodiment of the present invention;

FIG. 2 is a local sectional view schematically showing anelectrocatalytic tube according to the first embodiment of the presentinvention;

FIG. 3 is a diagram schematically showing the assemblage of theelectrocatalytic tubes according to a second embodiment of the presentinvention; and

FIG. 4 is a perspective view schematically showing an electrocatalytictube according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The technical contents of the present invention are described in detailin cooperation with drawings below.

Refer to FIG. 1 and FIG. 2 respectively a perspective view and a localsectional view schematically showing an electrocatalytic tube forcontrolling exhaust emissions according to a first embodiment of thepresent invention. The electrocatalytic tube 1 of the present inventionadopts to purify the exhaust containing nitrogen oxides, carbonmonoxide, hydrocarbons, and particulate matter. The electrocatalytictube 1 comprises a tube, an anode layer 20 and a cathode layer 30. Thetube is made of a solid-state electrolyte layer 10. The microstructureof the solid-state electrolyte layer 10 is a dense structure and made offluorite metal oxides or perovskite metal oxides, such as fluorite YSZ(Yttria-Stabilized Zirconia), stabilized zirconia, fluorite GDC(Gadolinia-Doped Ceria), doped ceria, perovskitestrontium/magnesium-doped lanthanum gallate, and doped lanthanumgallate.

The solid-state electrolyte layer 10 includes an inner wall 12 and anouter wall 13 opposite to the inner wall 12. The anode layer 20 iscoated on the inner wall 12. In one embodiment, the anode layer 20 ismade of a porous material, such as a cermet of nickel and fluorite metaloxides (e.g. a Ni-YSZ cermet), perovskite metal oxide, or metal-addedperovskite metal oxide. The cathode layer 30 is coated on the outer wall13. In one embodiment, the cathode layer 30 is made of a porousmaterial, such as perovskite metal oxide, fluorite metal oxide,metal-added perovskite metal oxide, or metal-added fluorite metal oxide(e.g. a perovskite lanthanum-strontium-cobalt-copper oxide, alanthanum-strontium-manganese-copper oxide, a combination of alanthanum-strontium-cobalt-copper oxide and a gadolinia-doped ceria, acombination of a lanthanum-strontium-manganese-copper oxide and agadolinia-doped ceria, a silver-added lanthanum-strontium-cobalt-copperoxide, a silver-added lanthanum-strontium-manganese-copper oxide, acombination of a silver-added lanthanum-strontium-manganese-copper oxideand a gadolinia-doped ceria, and a combination of a silver-addedlanthanum-strontium-manganese-copper oxide and a gadolinia-doped ceria).

The solid-state electrolyte layer 10 also includes an enclosed chamber11 enclosed by the inner wall 12 and having a reducing environment. Thereducing environment can have a sub-atmospheric pressure. In oneembodiment, the reducing environment has a pressure of ½-⅓ atm and has acarbon species or a reducing gas (such as carbon monoxide andhydrocarbons). Before the enclosed chamber 11 is sealed, carbon monoxideor hydrocarbons, such as methane, ethane, propane or propylene, isfilled into the enclosed chamber 11 to form a carbon species adhering tothe anode layer 20 and assisting in generating the electromotive force.Similarly, the reducing gas is filled into the enclosed chamber 11before the enclosed chamber 11 is sealed.

In one embodiment, the electrocatalytic tube 1 further comprises acatalytic oxidation layer 40 to assist in oxidation of the constituentsof the exhaust which are hard to oxidize on the cathode layer 30. Thecatalytic oxidation layer 40 adheres to the cathode layer 30 and is madeof a metal, an alloy, a metal oxide, a fluorite metal oxide, or aperovskite metal oxide, such as silver, palladium, platinum, orgadolinia-doped ceria.

Below is described the process of purifying the exhaust. Firstly, placethe electrocatalytic tube 1 of the present invention in an environmentof the exhaust. The exhaust is oxygen-rich or enriched with oxygen viaadding secondary air. The working temperature of the electrocatalytictube 1 is from ambient temperature to 600° C. The exhaust containsnitrogen oxides, carbon monoxide, hydrocarbons, and particulate matter.The purifying reactions undertaken by the present invention include thedecomposition reaction of removing nitrogen oxides and the oxidationreaction of removing carbon monoxide, hydrocarbons, and particulatematter.

Nitrogen oxides include nitrogen monoxide (NO) and nitrogen dioxide(NO₂). Nitrogen monoxide is decomposed into nitrogen and oxygen on thecathode layer 30. The reaction of NO decomposition is expressed byFormula (1):

2NO→N₂+O₂  (1)

Nitrogen dioxide is decomposed into nitrogen monoxide and oxygen on thecathode layer 30. The reaction of NO₂ decomposition is expressed byFormula (2):

2NO₂→2NO+O₂  (2)

Then, nitrogen monoxide is further decomposed into nitrogen and oxygenon the cathode layer 30.

The reducing environment of the enclosed chamber 11 and the oxidizingenvironment in the cathode layer 30 results in a difference of theoxygen partial pressure between the anode layer 20 and the cathode layer30 and thus generate an electromotive force (emf) between the anodelayer 20 and the cathode layer 30 according to Formula (3):

emf=[(RT)/(4F)]·ln[(P _(O2|Cathode))/(P _(O2|Anode))]  (3)

wherein R is the gas constant, T the absolute temperature, F the Faradicconstant, and P_(O2) the partial pressure of oxygen. The carbon speciesadhering to the anode layer 20 is a reducing compound to result in loweroxygen partial pressure over the anode and thus to generate largerelectromotive force. Different reducing gases and different reducingcompounds on the anode side result in different oxygen partial pressuresand thus generate different electromotive forces. Different oxygenconcentrations on the cathode side also result in different oxygenpartial pressures and thus generate different electromotive forces. Thehigher the oxygen concentration on the cathode side, the larger theelectromotive force and thus the higher promotion of decomposingnitrogen oxides into oxygen and nitrogen. Within a given temperaturerange, the lower the temperature, the higher the decomposition rate. Thedecomposition of nitrogen oxides can be effective at ambienttemperature.

As to eliminating carbon monoxide, hydrocarbons and particulate matterof the exhaust, the exhaust is oxygen-rich or enriched with oxygen viaadding secondary air. Then, the cathode layer 30 and the catalyticoxidation layer 40 convert carbon monoxide, hydrocarbons, andparticulate matter into harmless gases. For example, carbon monoxide isoxidized into carbon dioxide; hydrocarbons (HCs) and particulate matter(carbon-containing) are oxidized into carbon dioxide and water. Thereactions thereof are expressed by Formulae (4)-(6):

2CO+O₂→2CO₂  (4)

HCs+O₂→H₂O+CO₂  (5)

C+O₂→CO₂  (6)

Via the abovementioned catalytic decomposition reactions and catalyticoxidation reactions, the polluting constituents of the exhaust areeffectively removed.

Refer to FIG. 3 a diagram schematically showing the assemblage of theelectrocatalytic tubes for controlling exhaust emissions according to asecond embodiment of the present invention. In this embodiment, theelectrocatalytic tubes 1 are intermittently arranged according to therequirement for purification; between every two adjacentelectrocatalytic tubes 1 is formed a gas channel 2 where the exhaustpasses and contacts the cathode layer 30 of the electrocatalytic tube 1;thus is established a honeycomb-like structure 3 functioning as anelectrochemical-catalytic converter. Thereby, the present invention canpurify the exhaust with a compact assemblage of the electrocatalytictubes 1. In the honeycomb-like structure 3, one half of the channels aresealed to form the electrocatalytic tubes 1, and the other half of thechannels (i.e. the gas channels 2) are opened.

Refer to FIG. 4 a diagram schematically showing an electrocatalytic tubefor controlling exhaust emissions according to a third embodiment of thepresent invention. In this embodiment, the electrocatalytic tube 1 isfabricated to have a circular section. However, the present inventiondoes not restrict that the electrocatalytic tube 1 must have a circularsection. In the present invention, the electrocatalytic tube 1 may havea rectangular, pentagonal, or hexagonal section according to design.

In conclusion, the present invention uses the reducing environment ofthe enclosed chamber and the carbon species adhering to the anode layerto generate the electromotive force to promote the catalyticdecomposition reaction. Further, the present invention has simplestructure and lower fabrication cost. Furthermore, the reducingenvironment has a sub-atmospheric pressure, such as ½ atm, whereby thepresent invention is exempted from the structural damage caused bythermally-induced expansion and contraction, wherefore is prolonged theservice life of the present invention. Moreover, the electrocatalytictubes can be assembled to form a honeycomb-like converter, whereby thepresent invention has a compact assemblage able to be installed in theexhaust pipe of a vehicle engine for eliminating the pollutingconstituents of the exhaust and thus reducing air pollution.

Therefore, the present invention possesses utility, novelty andnon-obviousness and meets the condition for a patent. Thus, theInventors file the application for a patent. It is appreciated if thepatent is approved fast.

The embodiments described above are only to exemplify the presentinvention but not to limit the scope of the present invention. Anyequivalent modification or variation according to the spirit of thepresent invention is to be also included within the scope of the presentinvention.

What is claimed is:
 1. An electrocatalytic tube for controlling exhaustemissions, which adopts to purify the exhaust, comprising: a tube,composed of a solid-state electrolyte layer, wherein the solid-stateelectrolyte layer includes an enclosed chamber, an inner wall formedinside the enclosed chamber, and an outer wall formed outside theenclosed chamber, and wherein the enclosed chamber has a reducingenvironment; and an anode layer and a cathode layer respectively coatedon the inner wall and the outer wall of the solid-state electrolytelayer, wherein the reducing environment facilitates an electromotiveforce to occur between the anode layer and the cathode layer, andwherein nitrogen oxides of the exhaust are decomposed into nitrogen andoxygen on the cathode layer, and wherein carbon monoxide, hydrocarbonsand particulate matter of the exhaust are oxidized into carbon dioxideand water on the cathode layer.
 2. The electrocatalytic tube forcontrolling exhaust emissions according to claim 1, wherein the reducingenvironment is sub-atmospheric.
 3. The electrocatalytic tube forcontrolling exhaust emissions according to claim 1, wherein the anodelayer is made of a porous material.
 4. The electrocatalytic tube forcontrolling exhaust emissions according to claim 1, wherein the cathodelayer is made of a porous material.
 5. The electrocatalytic tube forcontrolling exhaust emissions according to claim 1, wherein the reducingenvironment includes a gas selected from a group consisting of carbonmonoxide and hydrocarbons.
 6. The electrocatalytic tube for controllingexhaust emissions according to claim 1, wherein a carbon species adheresto the anode layer.
 7. The electrocatalytic tube for controlling exhaustemissions according to claim 1, wherein the electrocatalytic tube has aworking temperature ranging from ambient temperature to 600° C.
 8. Theelectrocatalytic tube for controlling exhaust emissions according toclaim 1, wherein the anode layer is made of a material selected from agroup consisting of cermet of nickel and fluorite metal oxides,perovskite metal oxides, fluorite metal oxides, metal-added perovskitemetal oxides, metal-added fluorite metal oxides, and combinationsthereof.
 9. The electrocatalytic tube for controlling exhaust emissionsaccording to claim 1, wherein the cathode layer is made of a materialselected from a group consisting of perovskite metal oxides, fluoritemetal oxides, metal-added perovskite metal oxides, metal-added fluoritemetal oxides, and combinations thereof.
 10. The electrocatalytic tubefor controlling exhaust emissions according to claim 1, wherein thesolid-state electrolyte layer is made of a material selected from agroup consisting of fluorite metal oxides, perovskite metal oxides, andcombinations thereof.
 11. The electrocatalytic tube for controllingexhaust emissions according to claim 1, wherein a catalytic oxidationlayer is adhered to the cathode layer.
 12. The electrocatalytic tube forcontrolling exhaust emissions according to claim 11, wherein thecatalytic oxidation layer is made of a material selected from a groupconsisting of metals, alloys, metal oxides, fluorite metal oxides,perovskite metal oxides, and combinations thereof.
 13. Theelectrocatalytic tube for controlling exhaust emissions according toclaim 1, wherein the reducing environment has a pressure of smaller than½ atm.